Blood components such as haemoglobin are estimated using a colorimeter. The instrument consists of a box called a comparator. Inserted into it is a disc containing semi-transparent coloured windows. Blood chemically altered to colour different constituents, such as haemoglobin, is compared with the colour on the comparator. Therefore the concentration of the constituents is measured. This colorimeter was made by Tintometer Limited. It comes with three separate discs to measure carboxyhaemoglobin and haemoglobin A. Haemoglobin is the part of the blood responsible for carrying oxygen around the body. Carboxyhaemoglobin is carbon monoxide bonded to haemoglobin. This means that haemoglobin cannot transport oxygen around the body to vital organs.